Structure and method of manufacture

ABSTRACT

A structure for a chemical sensing device, the structure comprising at least one electrically conductive element located in, and protruding from, at least one recess. A method of manufacturing the structure includes: (a) providing a template comprising at least one recess having a recess depth; (b) providing an electrically conductive material in the at least one recess; and (c) removing part of the template to decrease the recess depth of the at least one recess, thereby forming said protruding at least one electrically conductive element.

BACKGROUND

A chemical sensing device may be used to determine the presence of atleast one certain chemical. Such a device may exhibit a known responsewhen exposed to a chemical, allowing the presence of the chemical to bedetected.

Known chemical sensing devices comprise sensing materials which exhibita change in a certain property, for example electrical conductivity,when they come into contact with certain chemicals. The sensitivity ofknown chemical sensing devices is limited by the detectability of thischange upon exposure to such chemicals.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate examples of the principlesdescribed herein and are a part of the specification. The illustratedexamples are merely examples and do not limit the scope of the claims.

FIG. 1 shows schematically an example of part of a chemical sensingdevice.

FIG. 2 shows schematically an example of part of a structure for achemical sensing device.

FIGS. 3a, 3b, 3c, 3d and 3e show schematically steps of an examplemethod of manufacturing a template for a structure for a chemicalsensing device.

FIGS. 4a, 4b, 4c, 4d and 4e show schematically steps of a method ofmanufacturing a structure for a chemical sensing device according to anexample.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present apparatus and methods. It will be apparent,however, to one skilled in the art that the present apparatus, systemsand methods may be practiced without these specific details. Referencein the specification to “an example” or similar language means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least that one example, but notnecessarily in other examples.

As described above, chemical sensing devices may be used to detect thepresence of at least one certain chemical by exploiting a known changein a certain property, for example the electrical conductivity, of acomponent of the sensing device when exposed to the chemical of interest(which may be referred to as the analyte).

Examples of a structure for a chemical sensing device will now bedescribed. Such structures comprise at least one recess and at least oneelectrically conductive element located in, and protruding from, the atleast one recess.

FIG. 1 shows part of an example chemical sensing device 1 comprising astructure 12 which comprises at least one recess and at least oneelectrically conductive element 4′, 4″ located in, and protruding from,the at least one recess. Each electrically conductive element issupported by the sides of a recess and by at least one substrate 8, 10as described in more detail later.

In the example shown in FIG. 1, a first electrically conductive element4′ and a second electrically conductive element 4″ are shown; thestructure in this example comprises more than two electricallyconductive elements; however, for clarity, only two electricallyconductive elements are shown. The extent of the structure shown isindicated by the dashed lines, but it is to be appreciated that theextent continues to the left and right of the Figure with further pairsof electrically conductive elements separated by a sensing material. Inother examples there may only be two electrically conductive elements.In the present example, the electrically conductive elements are ridgesextending along an axis perpendicular the plane of the page of FIG. 1,each ridge having a rectangular cross section. In another example, theelectrically conductive elements may form an array of interdigitatedelements; in other words, the electrically conductive elements may forma pattern of interlocking, yet electrically isolated, protrusions.

In this example, the first and second electrically conductive elements4′, 4″ are separated by an inter-element space 5. A sensing material 2is provided in the inter-element space 5 so as to at least partiallyfill the inter-element space 5 and to lie in contact with the first andsecond electrically conductive elements 4′, 4″. In this example, thesensing material 2 doesn't cover the electrically conductive elements4′, 4″ but, in other examples, the sensing material may cover at leastone of the electrically conductive elements and at least partially fillthe inter-element space; for example, as a film. The sensing material 2is chosen such that a certain property of the sensing material 2, forexample electrical conductivity, changes upon exposure to an analyte ofinterest. The sensing material is provided on a material 6 which issupported by the at least one substrate 8, 10 and which fills at leastpart of the inter-element space 5. The two electrically conductiveelements and sensing material may be considered as a resistor, with theresistance depending on the following equation:R=ρL/A,where R refers to the resistance of the resistor, ρ refers to theresistivity of the sensing material, length L refers to the separationbetween the two electrically conductive elements, and A refers to thecross-sectional area of the resistor in a direction orthogonal to thelength L.

In the example in which the electrical conductivity of the sensingmaterial 2 changes when it is exposed to the analyte, an electricallink, i.e. an electrical connection, is formed between the electricallyconductive elements 4′, 4″ via the sensing material 2. The conductivityof this link changes depending on the chemical environment it is exposedto. For example, if the sensing material 2 is exposed to the analyte ofinterest, the conductivity of the sensing material will change by aknown amount in dependence on the analyte concentration; this change ismeasurable.

Therefore, by monitoring the electrical properties of the electricallink, such as the resistance or electrical conductivity for example, thepresence of the analyte and in some examples also the concentration ofthe analyte can be determined. For example, if a detected change inresistance across the electrical link is equal to the expected change inresistance upon exposure to the analyte, this indicates that the analyteis present.

The electrical properties of the electrical link may be measured usingwell-known, commercially available measuring devices, for example anohmmeter, ammeter and/or voltmeter device or a multimeter such as theModel 2401 Low Voltage SourceMeter available from Keithley InstrumentsInc., Western Peninsula Building, Western Road, Bracknell, Berkshire,RG12 1RF, United Kingdom. The measuring device is connected to theelectrically conductive elements via electrical connections (not shown).In turn, the measuring device may be connected to a processing system,comprising for example at least one memory and at least one processor,for processing output signals of the measuring device, which outputsignals are indicative of the electrical property of the electrical linkbetween the elements. The memory may store data, for example in a lookup table, indicative of electrical property values, for exampleelectrical conductivity, or a change in electrical conductivity or arate of change of electrical conductivity, which corresponds with aknown concentration of analyte. Thus, a presence and concentration ofanalyte may be determined based on an electrical conductivity valuemeasured by the measuring device. The processor may also be connectedfor example to a display, for displaying data indicating that an analyteis present, and/or a concentration of analyte present. A variety ofdifferent materials may be used to form the sensing material 2. Inaddition to the material choice, a porosity of the sensing material tothe analyte affects performance of detecting the analyte. The density ofthe network of the applied sensing material may also need to be above apercolation threshold to the analyte, to enable conductivity between theelectrically conductive elements. In an example, the sensing material 2is selected from the group consisting of: a conductive organic polymer;inorganic nanoparticles coated with metal nanoparticles; inorganicnanoparticles coated with oxide nanoparticles; graphene; carbonnanotubes coated with metal nanoparticles; and carbon black.

More specifically, for example, a conductive organic polymer materialsuch as poly(thiophene)s (PT), poly(3,4-ethylenedioxythiophone (PEDOT)or poly(p-phenylene sulfide) (PPS), available from Sigma Aldrich Ltd.,The Old Brickyard, New Road, Gillingham, Dorset, SP8 4XT, UK, may beused for the sensing material 2. Alternatively, the sensing material 2may comprise inorganic metal or metal oxide nanoparticles, with suchmetals being for example platinum (Pt), rhodium (Rh), rubidium (Ru),platinum/rubidium (Pt/Ru), or platinum/cobalt metal (Pt/Co) availablefrom Sigma Aldrich Ltd. In a further example, carbon nanotubes, such astype ASP-100F single-walled carbon nanotubes, available from HanwhaChemical, Hanwha Building, 1 Janggyo-dong, Jung-gu, Seoul, South Korea,or graphene sheets, functionalized, for example coated, with metalnanoparticles, such as nickel, gold or palladium, may also be used;additional functionalization of these nanoparticles with molecularligands selective to a particular analyte can be achieved throughattachment of the ligand via thiol (—SH) functionality in the case ofgold (Au) and palladium (Pd) and silane coupling agents in the case ofnickel (Ni). The skilled person would readily understand methods forsuch functionalizing. Graphene sheets may be produced by solutionprocessing graphite flakes, available from Sigma Aldrich Ltd. forexample. The metal nanoparticles for functionalization can be depositedby electroless deposition, electrodeposition or vapour depositionchemicals for plating deposition, and may be purchased from EnthoneInc., Worldwide Headquarters, 350 Frontage Road, West Haven, Conn.06516, USA. In another example, nano wires, such as silver nano wiresavailable from Seashell Technology LLC, 3252 Holiday Ct. #115, La Jolla,Calif. 92037, USA, may be used. Alternatively, carbon black pigment,available from Hewlett-Packard Company, 3000 Hanover Street, Palo Alto,Calif. 94304-1185, USA, may be used; for example a commerciallyavailable black inkjet ink comprising carbon black pigment may be used,for inkjet printing the sensing material. The sensitivity of thechemical sensing device may depend on the number of possible electricalconnections within the electrical link described above; the number ofpossible electrical connections may depend on the pigment size of thecarbon black, which may be chosen appropriately to provide the desireddevice sensitivity. In a particular example, carbon black pigment isused to detect the presence of water vapor. The carbon black pigmentconducts electricity when dry, and the conductivity predictably changesin the presence of water vapor. Such a chemical sensing device incombination with a temperature measuring device to give temperature dataallows an accurate relative humidity measurement to be determined.

In an alternative example, the sensing material 2 is a semiconductormaterial, so the electrical link functions as a semiconductor. A changein the electrical conductivity of the electrical link may be measured todetect the presence of an analyte.

In the example illustrated in FIG. 1, the chemical sensing device 1comprises two electrically conductive elements 4′, 4″ forming a pair ofelements for detecting an analyte. The chemical sensing device 1 maycomprise more than one pair of electrically conductive elements, to forman array of pairs of elements. Each pair of elements may be electricallylinked by the same type of sensing material, or different pairs may beelectrically linked by different types of sensing material for detectingdifferent analytes. In each case, the array of elements will beconnected to a measuring device and processing system, similar to thatdescribed previously, but configured accordingly for an array as theskilled person would readily understand. In the case where each pair ofelements in an array is linked by the same type of sensing material, theprocessing system may be configured to determine an averageconcentration of analyte present over the entire array. In furtherexamples, there may be a plurality of arrays of elements, with eacharray forming a region of a larger chemical sensing device, with aplurality of regions, with each region being configured to detect thepresence of a different analyte.

Using FIG. 2, the example structure 12 will now be described in moredetail. For clarity of illustration, the sensing material is notillustrated in FIG. 2.

In the structure 12 of FIG. 2, each of the electrically conductiveelements 4′, 4″ are located in, and protrude from, a recess 13 in thematerial 6. The recess 13 is schematically indicated by dashed lines inthe Figure. It is to be appreciated that the sides of the recess 13 liein contact with the sides of the electrically conductive elements 4′,4″. Within a space between the protruding parts of the electricallyconductive elements may be placed a sensing material for sensing analyteof interest.

Each recess is an opening, in other words a hole or a cavity, having arecess depth 16 which defines the extent to which the recess extendsinto the material in which it is formed. Each recess also has a recesswidth 20, which is orthogonal to the recess depth 16. The recess widthdefines a lateral extent of a recess. The example recesses shown in FIG.2 are rectangular, with a constant recess width and recess depth,however, in other example recesses, the recess width and/or recess depthmay change throughout the recess. For example, recesses may have agreater width at the entrance to the recess, and a narrower width at thebase of the recess. It is to be noted that each recess extends along anaxis lying perpendicular to the plane of the page of FIG. 2. Therefore,in three dimensions, each recess is a channel for example.

The electrically conductive elements 4′, 4″ located within the recesses13 have an element length 14, illustrated as a vertical dimension inFIG. 2, and an element width 20, which is orthogonal to the elementlength 14 and is illustrated horizontally in FIG. 2. The element length14 in the example structure 12 is larger than the recess depth 16 suchthat each electrically conductive element 4′, 4″ protrudes from therecess it is located in. The difference between the element length 14and the recess depth 16 defines a protrusion length 22, in other wordsthe extent to which each electrically conductive element 4′, 4″protrudes from the recess it is located in. In the example shown in FIG.2, the electrically conductive elements 4′, 4″ are rectangular in crosssection, giving a constant protrusion length across the element width,and a constant element width across the protrusion length. However, inother examples, the electrically conductive elements 4′, 4″ may bedifferently shaped, provided they function to give a suitable electricallink via the sensing material for detecting an analyte.

In the example structure 12 shown in FIG. 2, the first electricallyconductive element 4′ comprises a first surface 24′ and the secondelectrically conductive element 4″ comprises a second surface 24″; thefirst surface 24′ and the second surface 24″ face each other and areseparated by an inter-element distance 18. This inter-element distance18 is measured in a direction orthogonal to the recess depth 16.

The sensitivity of a chemical sensing device depends on the surface areaof the sensing material available for exposure to an analyte. This inturn may depend on the volume of the inter-element space available forthe sensing material to occupy, which depends on the protrusion length22, a ratio of the protrusion length 22 to the inter-element distance 18and/or a ratio of the protrusion length 22 to the element width 20. Thevolume further depends on a second protrusion width, not shown, which istaken along an axis perpendicular the plane of the page of FIG. 2. Thesensitivity also depends on a porosity of the sensing material.

In example structures, the at least one electrically conductive elementprotrudes from the at least one recess with a protrusion length 22 in arange of: 1 to 100 micro-meters; 5 to 100 micro-meters; 10 to 100micro-meters; 15 to 100 micro-meters; 20 to 100 micro-meters; 25 to 100micro-meters; 30 to 100 micro-meters; 35 to 100 micro-meters; 40 to 100micro-meters; 45 to 100 micro-meters; 50 to 100 micro-meters; 55 to 100micro-meters; 60 to 100 micro-meters; 65 to 100 micro-meters; 70 to 100micro-meters; 75 to 100 micro-meters; 80 to 100 micro-meters; 85 to 100micro-meters; 90 to 100 micro-meters; 95 to 100 micro-meters; 1 to 20micro-meters; 5 to 20 micro-meters; 10 to 20 micro-meters; or 15 to 20micro-meters. A depth of the inter-element space between theelectrically conductive elements corresponds with the protrusion length.In examples, an aspect ratio of the protrusion length 22 to the elementwidth 20 is in the range of: 0.2:1 to 20:1; 0.5:1 to 20:1; 1:1 to 20:1;2:1 to 20:1; 3:1 to 20:1; 4:1 to 20:1; 5:1 to 20:1; 6:1 to 20:1; 7:1 to20:1; 8:1 to 20:1; 9:1 to 20:1; 10:1 to 20:1; 11:1 to 20:1; 12:1 to20:1; 13:1 to 20:1; 14:1 to 20:1; 15:1 to 20:1; 16:1 to 20:1; 17:1 to20:1; 18:1 to 20:1; 19:1 to 20:1; 0.2:1 to 2:1; 0.5:1 to 2:1; or 1:1 to2:1.

By locating the electrically conductive elements 4′, 4″ within the atleast one recess, a high aspect ratio may be achieved, since goodstructural support is provided to the electrically conductive elements4′, 4″ by the sides of the recess. In other words, since the surfaces ofthe electrically conductive elements 4′, 4″ within the recess are incontact with and abutting the recess sides, a base region of eachelement within the recess may be rigidly supported, meaning a greaterprotrusion length can be provided, and therefore a greater inter-elementspace volume for the sensing material. In example structures 12, theratio of the element length 14 to the recess depth 16 is such that theelectrically conductive elements 4′, 4″ are suitably structurallysupported; in some examples, the ratio of the element length 14 to therecess depth 16 is 1.1:1 to 2:1.

In the example structure 12, the first surface 24′ of one electricallyconductive element 4′ and the second surface 24″ of the otherelectrically conductive element 4″ are each planar and parallel orsubstantially parallel to each other. ‘Substantially parallel’ denotesthat the plane of the first surface and the plane of the second surfaceare parallel to each other within a range of +1-5 degrees.

The substantially parallel nature of the surfaces 24′ and 24″ in thisexample ensures a consistent and predictable performance for the sensingdevice, by providing a constant inter-element distance.

In the example in which the sensing material exhibits a change inelectrical conductivity when exposed to the analyte, the change inelectrical conductivity produces a corresponding change in resistance.Since the magnitude of the change in resistance when the analyte comesinto contact with the sensing material also depends on the separationbetween the electrically conductive elements, a constant separationbetween electrically conductive elements ensures a given change inelectrically conductivity corresponds with a known change in resistance,which is consistent across the length of the electrically conductiveelements. This is achieved in the example above by arranging the first24′ and second 24″ surfaces of the first 4′ and second 4″ electricallyconductive elements parallel to each other.

Examples of a method of manufacturing a structure for a chemical sensingdevice, such as that described above, will next be described.

In these examples, such a structure comprises at least one electricallyconductive element located in, and protruding from, at least one recess,as described previously. As an overview, the method of the examplesincludes:

(a) providing a template comprising at least one recess having a recessdepth;

(b) providing an electrically conductive material in the at least onerecess; and

(c) removing part of the template to decrease the recess depth of the atleast one recess, thereby forming the protruding at least oneelectrically conductive element.

An example method of manufacturing a template is illustrated in FIGS. 3ato 3e , using photolithography; other example methods include embossing,soft lithography, laser patterning, laser lithography and x-raylithography

In this example method, a layer of a first material 34, such as a thickfilm positive resist from the AZ range available from MicroChemicalsGmbH, Nicolaus-Otto-Str. 39, 89079 Ulm, Germany, or a resist from theSIPR range available from Shin-Etsu MicroSi Inc., 100028 S. 51^(st)Street, Phoenix, Ariz. 85044, USA, is deposited onto a substrate 26, asshown in FIG. 3a . This material may be deposited by, for example, spincoating, lamination or spray coating on the substrate 26.

Next, as shown in FIG. 3b , masks 30 are used to cover regions of thefirst material to remain after a developing step (in the example thatthe first material 34 is to act as a positive photoresist). Thus, themask pattern defines uncovered regions of the first material whichcorrespond with regions to form recesses in the template, as will beexplained later. Light 28 of an appropriate wavelength, for exampleultraviolet light with a wavelength of 436 nano-meters, 405 nano-metersor 365 nano-meters, is applied to the uncovered regions. The exposedfirst material may then be removed in a development step using anappropriate solvent such as aqueous alkaline developer, for exampletetramethlyammonium hydroxide (TMAH), available from Sigma Aldrich Ltd.,to form at least one recess 32 indicated using dashed lines in theFigure. These recesses 32 will be used to form the at least one recess13 of the structure described previously, as will be explained.

In other examples, a negative photoresist process may be used instead toform the recesses 32, using a mask to cover regions of the firstmaterial 34 to be removed.

In the next step, shown in FIG. 3c , a second material 6 is deposited asa layer onto the first material 34; the second material 6 may bedeposited by, for example, spin coating. In the example shown in FIG. 3c, the thickness of the layer of the second material 6 is the same as thethickness of the layer of the first material 34, however, in otherexamples, the first material 34 and the second material 6 may havedifferent thicknesses. As can be seen in the Figure, a layer of thesecond material 6 is formed in the recesses 32 also. In this example,the second material 6 will later form the supporting material for thesensing material of the structure; hence the same reference numeral forthe material is used as used previously when describing the structure12. The second material is different from the first material in thisexample.

Similar steps as described using FIG. 3b are then used for the secondmaterial 6, to expose certain areas of the second material 6 to light ofa given wavelength, such as ultraviolet light 28; this is shown in FIG.3d using a mask 30 with a different pattern from that used in FIG. 3b ,where the parts of the second material in the recesses 32 are coveredwith the mask. In this example, the second material is a negativephotoresist, such as SU-8 epoxy resin for example available fromMicroChemicals GmbH or from the MX series of dry film photoresistsavailable from DuPont (UK) Ltd., Wedgwood Way, Stevenage, Hertfordshire,SG1 4QN, UK. Exposure to light of an ultraviolet wavelength, such as 365nano-meters, will harden the exposed regions of SU-8 against removal bya solvent in a developing step.

In the final step of the present example, as shown in FIG. 3e , thesecond material 6 in the recesses 32 has been removed in a developmentstep by the action of an appropriate solvent, such as ethyl lactate orpropylene glycol methyl ether acetate (PGMEA), available from SigmaAldrich Ltd. or MicroChemicals GmbH to form a deeper recess 32 comparedwith that in FIG. 3b . In this example, both the first 34 and the second6 material are completely removed to form at least one recess 32,leaving the substrate 26 exposed. However, in other examples, only partof the first material 34 and/or the second material 6 may be removed,leaving the substrate 26 partly or entirely covered by part of the firstmaterial 34 and/or second material 6 in the recess. Each recess 32 maybe in the form of a hole, a cavity or any other opening in the first andsecond materials 34, 6.

The template, manufactured according to the methods described withreference to FIG. 3 for example, can then be used to manufacture astructure for a chemical sensing device. This is now described usingFIGS. 4a to 4 e.

In FIG. 4a , as an example of step (a) of the overview method givenabove, a template is provided which comprises at least one recess 32. Inthe example shown in FIG. 4a , the template is that described previouslyusing FIGS. 3a to 3e , and therefore comprises a first layer of a firstmaterial 34 and a second layer of a second material 6. In otherexamples, the template may comprise a single layer of one material. Infurther examples, the template may comprise more than two layers ofdifferent materials. At a first side of the template there is providedthe first substrate 26 in contact with the first side.

A next step is shown in FIG. 4b . In this step, as an example of step(b) of the overview method above, an electrically conductive material4′, 4″ is provided in the at least one recess 32. The at least onerecess 32 is now indicated in the Figure using dashed lines. In anexample, this step includes using electroplating of for example nickel,but in other examples copper or other platable metals may be used.Suitable materials for electroplating are available from Enthone Inc. Inthe example in which electroplating is used to deposit the electricallyconductive material in the at least one recess, the first substrate 26may be electrically conductive, such as stainless steel or a stainlesssteel coated substrate, for providing an electric current to enable theelectroplating process. As an example, electroplating may be performedby providing the at least one recess in a nickel plating bath includingfor example 400 milli-liters of nickel sulphate per liter, 200milli-liters of nickel chloride per liter and 30 grammes of boric acidper liter, at a temperature of 58 degrees celsius. By applying a currentof 0.5 amps to the electrically conductive first substrate 26, a platingrate of 100 amp seconds per micron may be achieved. Brighteners mayoptionally be added to the electroplating solution. In other examples,other methods may be used to provide the electrically conductivematerial 4′, 4″ in the at least one recess, such as electroless metaldeposition.

Since the electrically conductive material will form the at least oneelectrically conductive element of the structure, the same referencenumerals are used for the material as used previously for theelectrically conductive elements.

In an example, the electrically conductive material is provided so as tofill the at least one recess 32. The filled at least one recess 32corresponds with the filled recess 13, described later with reference toFIG. 4d . In filling the at least one recess, the electricallyconductive material abuts the sides of the at least one recess. Thiscontact between the electrically conductive material and the recesssides will provide good structural support for the electricallyconductive elements of the structure being manufactured, allowing agreater aspect ratio to be provided.

In FIG. 4c , a second substrate is placed in contact with the templateand covers the at least one recess 32, for example using a laminationprocess. The second substrate is provided on a second side of thetemplate, opposite the first side. In this example, the second substratecomprises an adhesive layer 8, such as Norland 81 Optical Adhesive,available from Norland Products Inc., 2540 Route 130, Suite 100,Cranbury, N.J. 08512, USA, and a further substrate layer 10, such aspolyethylene terephthalate (PET), available from DuPont Teijin FilmsU.S. Limited Partnership, 3600 Discovery Drive, Chester, Va. 23836, USA.In other examples, the second substrate may comprise one layer, or morethan two layers of different materials. Since the adhesive and PETsubstrate layers in the present example structure of FIG. 2 will formthe substrate layers of the structure being manufactured, they arereferred to using the same reference labels 8, 10 as previously usedwhen describing the structure of FIG. 2.

In a further step of this example, the first substrate 26 is removed,for example using a peel-off process. The resulting structure is shownin FIG. 4d . The recess 32 is now labeled with the reference numeral 13,in keeping with the reference numerals of the structure of FIG. 2.

In a final step of the example, illustrated in FIG. 4e and as an exampleof step (c) of the overview method above, part of the template isremoved, from the second side, to decrease the recess depth of the atleast one recess 13, thereby forming the protruding at least oneelectrically conductive element 4′, 4″ of the structure. The structuremay have been inverted before or after removing the first substrate 26,and before removing the part of the template from the second side.

In more detail, the part of the template removed is the first layer ofthe first material 34, leaving the layer of the second material 6remaining. Thus, the first material 34 acts as a sacrificial material.An etching process may be used for the removal, such as oxygen plasmaetching, or plasma etching with oxygen combined with tetrafluoromethane(CF₄). Alternatively, a solvent such as N-methyl-2-pyrrolidone (NMP),available from Sigma Aldrich Ltd. can be used to remove one layer ofresist. With the first layer having a first thickness and the secondlayer having a second thickness, and the part of the template removedbeing the first layer, the decreased recess depth corresponds to thesecond thickness. Therefore, the protrusion length of the at least oneelectrically conductive element corresponds with the thickness of thefirst material 34. In other examples, the method may include removingpart of a layer to protrude the electrically conductive elements. Forexample, where the template comprises a single layer, instead of thefirst and second layers, the method may include removing part of thesingle layer, leaving a remaining part in contact with the secondsubstrate to form the at least one recess.

Once these steps have been completed, a structure for a chemical sensingdevice in accordance with that example described previously has beenmanufactured. A sensing material may then be provided in theinter-element spaces between the electrically conductive elements, forforming a chemical sensing device. The sensing material may be appliedas a liquid based conductive material, using for example inkjet printingor spray coating. Controlling of a droplet size can be used to determinethe network structure and/or density of the sensing material. As theskilled person will understand, electrical connections may also beformed to connect each electrically conductive element to a processingsystem and a measuring device, in accordance with the examples describedabove. These electrical connections may be formed after forming thestructure, or the above method may be modified to include forming of theelectrical connections, as the skilled person would understand. Examplesof a structure for a chemical sensing device, and an associated methodof manufacture, have been described. The structure according to theexamples improves sensitivity of a chemical sensing device compared toknown structures, since the method of manufacture allows electricallyconductive elements with a greater protrusion length, and therefore agreater surface area for contact with a sensing material, to be formed.Moreover, the method of manufacture is more cost efficient than methodsusing for example isotropic silicon etching. The method in examplesusing the combination of photolithography and electroplating toelectroplate metallic elements using a template, to form the structure,is simple and effective. Further, the method is suitable for use withflexible substrates, enabling a structure made according to examplesherein to be used in a roll to roll manufacturing process for flexibleelectronic devices.

The preceding description has been presented only to illustrate anddescribe examples of the principles described. This description is notintended to be exhaustive or to limit these principles to any preciseform disclosed. Many modifications and variations are possible in lightof the above teaching, in accordance with the scope of the appendedclaims.

The invention claimed is:
 1. A method of manufacturing a structure for achemical sensing device, the structure comprising a plurality ofelectrically conductive elements located in, and protruding from, aplurality of recesses in the structure, the method comprising: providinga first substrate; forming a first layer on the first substrate; forminga second layer on the first layer; forming the plurality of recesses,wherein the plurality of recesses extend through the second layer and atleast partially through the first layer; providing an electricallyconductive material in the plurality of recesses; providing a secondsubstrate in contact with the second layer and covering the electricallyconductive material; and removing the first substrate and the firstlayer without removing the second layer to decrease the recess depth ofthe plurality of recesses, thereby forming said plurality ofelectrically conductive elements between portions of the second layer.2. The method according to claim 1, wherein the first layer comprises afirst material and the second layer comprises a second, different,material, wherein the first layer has a first thickness and the secondlayer has a second thickness, and wherein the decreased recess depthcorrespond to the second thickness.
 3. The method according to claim 1,wherein providing the electrically conductive material includes usingelectroplating to provide the electrically conductive material in theplurality of recesses.
 4. The method according to claim 1, whereinproviding the electrically conductive material further comprisesproviding the electrically conductive material so as to fill theplurality of recesses.
 5. The method according to 4, wherein theelectrically conductive material is provided within the plurality ofrecesses so that surfaces of the electrically conductive material abutthe plurality of recesses.
 6. The method according to claim 1, furthercomprising: providing a sensing material on the second layer followingremoval of the first layer such that the sensing material is in contactwith the electrically conductive material located in and protruding froma plurality of recesses.
 7. The method according to claim 6, whereinproviding the sensing material further comprises providing the sensingmaterial on the second layer between the electrically conductivematerial extending above the second layer such that the electricallyconductive material extends above the sensing material.
 8. The methodaccording to claim 6, wherein the sensing material is selected from thegroup consisting of: a conductive organic polymer; inorganic metalnanoparticles; inorganic oxide nanoparticles; graphene sheets coatedwith metal nanoparticles; carbon nanotubes coated with metalnanoparticles; nano wires and carbon black.
 9. The method according toclaim 6, wherein an electrical conductivity of the sensing material isto change upon exposure of the sensing material to an analyte ofinterest.
 10. The method according to claim 1, wherein forming theplurality of recesses further comprises forming the plurality ofrecesses to have at least a predetermined distance between the pluralityof recesses.